In the manufacture of fuel rods intended to form assemblies for nuclear reactors, for pressurized water nuclear reactors in particular, pellets of fuel are introduced into a metal tube called a can and this tube is sealed at each of its ends by plugs welded sealingly at the ends of the tube after the air inside the tube has been evacuated and helium under sufficient pressure has been introduced.
The sealed fuel rod thus contains an inert gas under pressure which remains inside the canning for the whole life of the fuel rod. The presence inside the fuel rod of materials which attack the nuclear fuel or the canning material or, furthermore, materials liable to undergo nuclear reactions leading to the production of dangerous radio-active products is thus avoided.
The fuel rods are assembled in the form of parallel bundles by means of connecting structures allowing the formation of assemblies which will be introduced into the core of the nuclear reactor. If moisture has been introduced inside the fuel rod during its manufacture, the canning of this rod is liable to be hydrided during the working of the reactor.
It is therefore extremely important to confirm that these rods do not contain the least trace of moisture or water before the fuel rods are assembled.
This testing must be carried out from outside a sealed fuel rod and must be conducted with very great sensitivity.
Known methods also exist which allow the detection, inside a fuel assembly, of defective fuel rods, i.e. whose canning has fissures which can cause radio-active products to issue into the cooling fluid of the nuclear reactor.
The detection and locating of these defective fuel rods inside assemblies allows them to be replaced and allows the assembly whose defective rods have been replaced to be then used.
To detect these defective fuel rods, methods have been proposed using the propagation of ultrasounds in the can of the fuel rod and the measurement of the attenuation of the ultrasounds which may arise from the presence of faults in the material of the canning. This testing is carried out on assemblies which have been in service in the core of the reactor for a certain period of operation and the faults detected by the attenuation of the ultrasounds are constituted by fractures in the can introducing a significant attenuation factor.
The introduction of cooling water inside the fuel rods also causes very considerable attenuation of the ultrasonic signal because of the relatively large quantities of water introduced into the fuel rod through the fissures in its canning.
A method of locating defective fuel rods is also known in which the cans of the fuel slugs are heated near one plug so that, when a defective rod is present, the moisture inside this rod is vaporized and causes bubbles of vapour or drops of condensation to form which can be detected at the level of the plug by means of an ultrasonic echo test.
In all these methods of detecting defective fuel rods inside assemblies which have been in service, the propagation of ultrasounds in the can of the fuel element is effected from an accessible region of the fuel rod in the assembly, i.e. from the plug of this fuel element or from the region immediately adjacent to the plug. In fact, the remainder of the fuel rod is not accessible in the assembly from which the end plates have been removed except where fuel rods disposed at the periphery of the assembly are concerned.
These tests must also be made on the assembly immersed inside the swimming pool of the reactor since we are concerned with an assembly which has remained in the core of the nuclear reactor for some time.
For all these reasons, the ultrasonic testing methods used hitherto for detecting fuel rods have not been transferable to testing for very slight traces of moisture in new fuel rods where the can has no fissures or other significant faults.
A method of testing steam generator tubes is also known, which uses the propagation of ultrasonic waves produced by the reflection of ultrasounds at the wall of these thin tubes, termed "plate waves" or "Lamb waves". These Lamb waves undergo attenuation when they encounter a fault such as a fissure or a saw cut in the tubular wall.